The present invention is directed generally to inkjet inks, and, more particularly, to inkjet inks having improved lightfastness.
Inkjet inks have been known for some time, and have been commercialized by a number of manufacturers, including Hewlett-Packard. Many issues have been addressed over the years, such as kogation, water fastness, and smear fastness. Improvements in ink compositions have led to improved resistance to kogation and improved water fastness and smear fastness, among other properties.
With many of the basic problems overcome, or at least alleviated, attention has turned to providing ink formulations with improved lightfastness. Lightfastness is a measure of the stability of the ink when exposed to ambient light. Many of the colorants (dyes and pigments) tend to fade upon prolonged exposure to light, particularly ultraviolet (UV) light.
Accordingly, it is desired to provide an ink formulation having improved stability to light, that is, able to withstand fading upon exposure to ambient light, so that the print, whether black or color, remains virtually unchanged over time.
In accordance with the present invention, lightfast additives are provided. These additives exhibit excellent lightfastness with many dyestuffs (both water-soluble and water-insoluble). Thus, the lightfast additives work with both dyes (water-soluble and water-insoluble) and pigments (which are water-insoluble). While pigments are known for their lightfastness, compared to dyes, nevertheless, use of the additives of the present invention can further improve the lightfastness of pigments.
The lightfast additives of the present invention exhibit good water solubility and significant effect on dye and pigment molecules which fade most when exposed to UV, ambient light, etc., and are suitable for improving permanence of inkjet inks, including thermal inkjet ink, piezo inkjet ink, and the like.
According to the present invention, the molecule which can improve the lightfastness of the dyes is composed of three parts or moieties:
Axe2x80x94Xxe2x80x94Bxe2x80x83xe2x80x83(I)
where:
moiety A contributes to the water solubility and compatibility (molecular overlapping) with dye molecule in solid state;
moiety B contributes to improvement of lightfastness of the dye; and
moiety X is a connector between A and B.
By xe2x80x9ccompatibilityxe2x80x9d is meant that the dye or pigment molecules and the lightfast additive molecules have a similar or partly similar structure so that they can xe2x80x9coverlapxe2x80x9d effectively. By xe2x80x9cmolecular overlapxe2x80x9d is meant that molecules with similar structures mix very well in solution. They maintain homogeneity. Even in the solid state, this homogeneity is maintained when the solvent is removed. That is to say, there is no phase separation, either in the liquid state or in the solid state.
In the above molecule (I), moiety A is an aryl sulfonic acid metal salt with or without substituents, for example, benzene sulfonic acid, benzene sulfonic acid sodium salt, hydroxybenzenesulfonic acid sodium salt, naphthalene mono or di sulfonic acid, acid form and salt form, moiety X is xe2x80x94NRxe2x80x94, xe2x80x94Oxe2x80x94, and moiety B is a triazine, benzotriazole, tetrazole, or benzimidazole, with or without substituents.
The Axe2x80x94Xxe2x80x94B molecules where B is a triazine can be prepared by a one-step reaction of reactive triazines (e.g., cyanuric chloride, melamine) with reactive aryl sulfonic acid (amino, diamino). For Axe2x80x94Xxe2x80x94B molecules where B is a benzotriazole, a tetrazole, or a benzimidazole, alternate routes are used.
In any event, the lightfastness of inkjet inks containing dyes or pigments is improved, compared to inks not containing the additive of the present invention.